FBI(WASHINGTON) -- Investigators believe Aaron Alexis, a former Navy reservist, was the lone gunman in the shooting spree at the Navy Yard in Washington, D.C., Monday, in which 12 people were killed before the suspect was killed in a firefight with police. Authorities have lifted a shelter in place for the remaining residents in the area.

The 12 deceased shooting victims range in age from 46 to 73 years old, Washington D.C. Mayor Vincent Gray said at an evening news conference on Monday. Officials are still notifying the families of about half of the 12 people who were killed, he said.

"We have no evidence that any active military are amongst the victims," Metropolitan Police Chief Cathy Lanier said.

Officials said three others suffered gunshot wounds and five more people suffered other injuries.

The death of Alexis, 34, brought the toll of the carnage at the Navy Sea Systems Command headquarters to 13. The shooter's identity was confirmed based on a partial fingerprint analysis, authorities said.

The suspect had a security clearance that allowed him onto the Navy Yard as part of his civilian subcontracting work, officials said.

Alexis and members of the subcontracting team, according to law enforcement sources, were staying at a Residence Inn about a mile from the Navy Yard. The suspect's car was found on the Washington Navy Yard, law enforcement sources said.

A senior law enforcement official said he used his security clearance to get on campus and it appeared he did not force his way onto the property. Officials were in the process of getting a search warrant to search the vehicle, which was described as a rental car.

Authorities had earlier said they were searching for a possible second suspect, but officials said at the evening news conference the search for a black suspect in olive-drab uniform age 40 to 50 has been exhausted.

Earlier Monday, Lanier praised the works by police officers who responded to the incident.

"I think the actions by the police officers, without question, helped to reduce the number of lives lost," she said.

Lanier said police and the suspect exchanged gunfire "multiple times" before he was shot and killed in a final gun battle.

Valerie Parlave, assistant director in charge of the FBI's Washington, D.C. field office, said investigators were trying to learn everything they can about Alexis.

"No piece of information is too small," Parlave said.

She asked anyone with information about his recent movements, contacts and associates to contact the FBI at 1-800-CALL-FBI.

Another man who was sought for a possible connection to the shooting was located Monday afternoon and was no longer a "suspect or person of interest," according to a tweet from the Metropolitan Police Department.

Lanier declined to discuss what evidence led police to believe the massacre could have potentially been carried out by more than one person.

Among the wounded was a law enforcement officer who was shot in an exchange with the gunman.

The shooting brought parts of Washington, D.C. to a standstill.

The Washington Nationals, whose stadium is a couple of blocks away from the Naval Sea Systems Command headquarters, said the team's game Monday night against the Atlanta Braves was postponed until Tuesday.

The Senate complex was placed under a temporary lockdown Monday afternoon "in light of the uncertainty surrounding the shooting" and the possibility that a second shooter might have been at large, Senate Sergeant at Arms Terrance W. Gainer wrote in a note to the Senate community.

At nearby Reagan National Airport Monday morning, a ground stop was imposed by the FAA. All planes have since resumed flying out of the airport, a Metropolitan Washington Airports Authority spokeswoman told ABC News.

Nine D.C. public schools were placed on lockdowns Monday morning, according to the District of Columbia Public Schools' Twitter account, but all schools were dismissed as scheduled Monday afternoon.

Washington D.C. Rep. Eleanor Holmes Norton told reporters the district has not "had a day like this since 9/11."

President Obama ordered the flags at all federal and military installations to be flown at half-staff through Friday in honor of the victims.

At the beginning of a news conference Monday, Obama said he was briefed on the shooting.

"We are confronting yet another mass shooting, and today it happened on a military installation in our nation's capital," he said.

Obama said he wanted a "seamless" investigation into the shooting and was standing with the victims and their families affected by what he called a "cowardly act."

"It targeted our military and civilian personnel, men and women who were going to work, doing their job protecting all of us. They are patriots and they know the dangers of serving abroad," Obama said. "But today they faced the unimaginable violence that they won't have expected here at home."